1. Technical Field
A method for formation of conductive polymers using an in situ generated nitrosyl ion as an oxidizing agent is disclosed. Nitrosyl ion is generated either electrochemically or chemically. Application of the resulting polymers and polymer-inorganic composite materials thus generated in various areas (e.g., energy conversion/storage, coatings, sensors, drug delivery, and catalysis) is also disclosed.
2. Description of the Related Art
Conducting polymers combining the desirable features of organic polymers and electronic properties of semiconductors are attractive materials for use in energy conversion/storage, optoelectronics, coatings, and sensing technologies. In general, polymerization of conducting polymers is initiated by chemical or electrochemical oxidation of monomers to radicals, followed by radical coupling and chain propagation. While chemical oxidation involves the use of oxidizing agents, such as FeCl3, electrochemical oxidation is typically achieved by applying an anodic bias (bias that causes oxidation reaction to occur at the working electrode) to a conducting substrate immersed in a monomer solution (anodic electropolymerization). The electrochemically initiated polymerization is generally used to prepare film- or electrode-type conducting polymers, as it localizes polymerization to working electrodes with convenient control over film thickness and morphology.
In a further development, conducting polymers have been utilized as a matrix to embed or disperse metal particles (e.g., Cu, Au, Ag, Ni, Ru, Ir, Pt, Co, Pd, Fe) to form conductive polymer-metal composite electrodes for use in various electrochemical applications (e.g., sensors and electrocatalysts). Typically, these hybrid electrodes are prepared by a two-step electrodeposition process: electropolymerization (anodic deposition) followed by metal deposition (cathodic deposition). This two-step process not only makes the preparation cumbersome and expensive but also limits the types and qualities of the metal-polymer composite thus generated. However, because anodic electropolymerization and cathodic metal deposition require an oxidation and a reduction reaction at the working electrode, respectively, with significantly different and often incompatible ranges of potentials, one-step process for preparing metal-conducting polymer hybrid films has yet to be developed.
Another important class of conducting polymer-based composite materials can be prepared when a conductive polymer is combined with high surface area mesoporous silica materials. Mesoporous silica materials have been utilized for various applications (catalysis, sensing, drug delivery, adsorption and separation) due to their uniform mesoporous features as well as high surface areas. When a conductive polymer layer is deposited on the mesopore walls, the physicochemical properties as well as the surface nature of the silica (e.g. hydrophilicity and surface charge) can be modified, which allows for adsorption and/or immobilization of a wide range of molecules/species on the mesopore walls, thereby significantly broadening the application of the mesoporous materials. In addition, a conductive polymer coating may convert the insulating mesoporous silica materials into semiconducting composites that can be used for sensors and electrocatalysis.
In order to obtain silica-polymer composites retaining uniform and accessible mesopores, a thin polymer coating should be introduced on the mesopore walls in a uniform manner without clogging the mesopore entrances. When monomers and initiators (e.g. oxidizing agents) are mixed with mesoporous silica particles in one reaction chamber, polymerzation occurs predominantly in bulk solution or on the surface of silica particles because the diffusion of monomers or initiators into the pores is less favored. This clogs the pore entrances and hinders the formation of high quality composite mesoporous particles, and/or creates an undesirable mixture of pure polymer particles and composite particles in solution.
To achieve desirable polymerization within the mesopores, several approaches have been developed, which commonly require a two-step procedure. Specifically, monomers are first adsorbed within the silica mesopores, and are then transferred to a different chamber to be mixed with initiators. Because the interaction between the monomers and initiators in the solution phase is limited, undesirable bulk polymerization may be significantly suppressed. Again, although such two-step processes make the preparation cumbersome and expensive, one-step formation and deposition of conductive polymers on mesoporous silica walls has yet to be developed.